Oddly enough, she wasn’t. An “Erika”-style character was supposed to show up earlier, in EP3, in the scrapped Land of the Golden Witch. The name of that character was Virgilius, although with a slightly different role. He was supposed to be a second detective, with a different perspective from Battler, arguing against his reasoning.
If you want to check this information, go to Virgilia’s page on the umineko wikia, in the trivia section. Or check the “Answer of the Golden Witch”.
If anything, the decision to include Virgilius after all is what might’ve been a shorthanded.
Mind, I agree that Erika appeared too late and added too little to the mystery portion. Aside that I appreciate her as a parody, or deconstruction as UsagiTenpura put it, of the detective character. And her over-the-top offensive behavior also helped slowly shift the perspective of the reader: while Beatrice in EP2 (and the beginning of EP3) is the epitome of the evil witch, Erika is the epitome of the “evil truth/truthseeker” (… I am bad with naming things, you get what I mean).
She just needed to appear earlier. Oh Land of the Golden Witch, you are missed.
… also she reminds me of someone I know personally. Sharp intellect, really good at discussions, but overall doesn’t even care what he is discussing about - only winning the discussion matters. And he’ll go to any lengths for that.